1. Field of the Invention
The present invention is related to voltage trim amplifiers used in programmable memory devices and specifically to integrated voltage trim amplifiers with MOS feedback elements.
2. Art Background
In the field of programmable memory devices it is common to maintain high voltage regulation by using a feedback amplifier configuration to provide multiplication of an input voltage by quantized values. The multiplying feedback network typically works with an operational amplifier ("op-amp") as the active gain element and accomplishes the function through a conventional resistor network as shown in FIG. 1. Referring to FIG. 1, the voltage-series feedback amplifier comprises an op-amp 150 having a non-inverting terminal 101 for input voltages and an inverting terminal 102 for the feedback voltage.
To help describe the present invention, the theory of the feedback amplifier is discussed as follows. Because the potentials of the two input terminals to a high gain op-amp are virtually identical, the gain of this amplifier configuration can be expressed as: EQU GAIN=Vout/Vin=Vout/VR2=(R1+R2)/R2; EQU Therefore, EQU Vout=Vin*(1+R1/R2). (Equation 1)
As shown in the equation, the gain of the amplifier can be made variable by varying the ratio of the feedback resistors. This variability is quite useful in the art of programmable memory products in that the output voltage from the amplifier could be made dependent upon the ratioed resistance of the feedback network. However, there are problems associated with using resistors in the integrated circuit devices. First, in advanced CMOS processes, high value quality resistors are hard to come by. Secondly, resistors in a monolithic integrated circuit take up much more area in the silicon than MOSFETs. Also, practical resistors cannot operate at very high impedance to enable the circuit to run on reduced currents. Further, the resistors are not stable over process and temperature parameters.
As will be described in the following, the present invention is an amplifier feedback circuit where the entire circuit can realized using MOS devices, a feature that is compatible with the advanced CMOS processes nowadays. Also, by using MOS devices the silicon usage is greatly reduced as opposed to the integrated resistors. The feedback transistors in the amplifier of the present invention can potentially operate at very high impedance compared to what is practical with resistors. Therefore, the output impedance of the op-amp can be made higher and the entire circuit can be run on reduced currents.